The present invention relates to a catalyst comprising alkali metal tungstate for the synthesis of alkyl mercaptans from alkanols and hydrogen sulphide, and to a process for preparing this catalyst.
In this patent application, the term alkali is understood to mean the bound alkali metals of the Periodic Table of the Elements or mixtures of at least two alkali metals bound in the tungstates. In this case, caesium occurs only together with a further element of the alkali metal group.
Methyl mercaptan in particular is an industrially important intermediate, for example for the synthesis of methionine and for the synthesis of dimethyl sulphoxide and dimethyl sulphone. It is nowadays prepared predominantly from methanol and hydrogen sulphide by reaction over a catalyst composed of aluminium oxide. The methyl mercaptan is synthesized commonly in the gas phase at temperatures between 300 and 500° C. and at pressures between 1 and 25 bar.
In addition to the methyl mercaptan formed, the reaction mixture comprises the unconverted starting materials and by-products, for example dimethyl sulphide and dimethyl ether, and also the gases which are inert for the purposes of the reaction, for example methane, carbon monoxide, hydrogen and nitrogen. The methyl mercaptan formed is removed from this reaction mixture.
For the economic viability of the process, a maximum selectivity is required in the catalytic reaction of methanol and hydrogen sulphide to give methyl mercaptan in order to keep the removal of the methyl mercaptan formed from the reaction mixture as uncomplicated and inexpensive as possible. Here, especially the energy demands for the cooling of the reaction gas mixture to condense the methyl mercaptan constitute a large cost factor.
To increase activity and selectivity, aluminium oxide as a support is typically admixed with potassium tungstate or caesium tungstate. In this case, the tungstate is commonly used in amounts up to 25% by weight based on the total weight of the catalyst. An improvement of activity and selectivity is also obtained by increasing the molar ratio of hydrogen sulphide to methanol. Typically, molar ratios between 1 and 10 are employed.
However, a high molar ratio also means a high excess of hydrogen sulphide in the reaction mixture and thus the need to conduct large amounts of gas in circulation. To reduce the energy demands required for this purpose, the ratio of hydrogen sulphide to methanol should therefore deviate only slightly from 1.
U.S. Pat. No. 2,820,062 relates to a process for preparing organic thiols, in which a catalyst composed of active aluminium oxide which is admixed with potassium tungstate in an amount of 1.5 to 15% by weight, based on the weight of the catalyst, is used. With this catalyst, good activities and selectivities are achieved at reaction temperatures of 400° C. and molar ratios of 2. This US patent mentions various possibilities for the introduction of the potassium tungstate into the aluminium oxide. For instance, it is said to be possible to employ impregnation processes, coprecipitations and pure mixtures. Little significance is attributed to the actual preparation of the catalyst for the economic viability of the synthesis process of methyl mercaptan.
EP 0 832 687 B1 describes the advantages of the use of caesium tungstate (Cs2WO4) instead of potassium tungstate (K2WO4) as a promoter. For instance, use of caesium tungstate can achieve an enhanced activity with simultaneously good selectivity.
Increasing the caesium tungstate concentration to up to 40% by weight allows the selectivity for methyl mercaptan to be increased to 92% without the activity being disproportionately worsened.
According to the general view, the best selectivity is achieved with catalysts for which the alkali metal/tungsten ratio is equal to 2:1 (A. V. Mashkina et al., React. Kinet. Catal. Lett., Vol. 36, No. 1, 159-164 (1988).